Combustion in the cylinders of an internal combustion engine generates waste heat which must be removed to allow proper operation of engine components. The engine valves and valve seat inserts are two such component exposed to the high combustion temperatures. The valve seat inserts are typically made of material with thermal properties which accommodate valve expansion and contraction. Unless the valve seat inserts are properly cooled, however, the valves may not seat properly potentially resulting in engine "knock".
In most engines, conductive cooling of the valve seat inserts, which is the dominant heat transfer mechanism, is accomplished through heat conduction to the adjacent cylinder head in which it is fitted. Although closely toleranced, voids typically exist between the valve insert and the cylinder head. These voids raise the thermal contact resistance between the insert and the head which can result in a significant valve insert temperature rise, perhaps exceeding 100.degree. F.
Use of a metallic coating on the valve insert which melts at engine operating temperatures was disclosed in U.S. Pat. No. 2,517,114 (Karcher) to improve the thermal contact between the insert and the cylinder head. Such an assembly is impractical, however, as the metals with the highest thermal conductivity, such as aluminum, copper, silver or gold, do not melt at normal engine operating conditions. Metals such as sodium or lead may not be compatible with the material used to make the insert, typically a powdered metal. In addition, repeated phase changes of the metallic coating could lead to unequal distribution of the coating upon resolidification resulting in localized "hot spots" due to insufficient thermal conduction.